Life Cycles of Stars

Low- to Medium-Mass Stars Scientists can determine the masses of stars by observing the gravitational interaction of stars that occur in pairs. Based on these observations, scientists have determined that there is a relationship between mass and brightness.

High-Mass Stars Scientists can determine the masses of stars by observing the gravitational interaction of stars that occur in pairs. Based on these observations, scientists have determined that there is a relationship between mass and brightness.

Nebula High and Low/ Medium

Protostar High and Low/ Medium

Adult Star High and Low/ Medium

Red Giant Low/ Medium

Red Supergiant High

Planetary nebula Low/ Medium

White Dwarf Low/ Medium

Black Dwarf Low/ Medium

Supernova High

Neutron Star High

Black Hole High

A large cloud of interstellar gas and dust spread out over a large volume of space. What does interstellar mean? Discuss the answer to this question with your group and record your ideas on a sheet of notebook paper.

Stars are created by gravity Stars are created by gravity! Gravity pulls interstellar gas and dust together into a denser cloud. As gas and dust collapse inward due to gravity eventually there is enough mass to form a star.

When the contracting cloud of and dust becomes so dense and hot that nuclear fusion begins, an this is formed! About 90% of a stars life is spent in this phase on the main sequence. In the core of all stars, nuclear fusion converts hydrogen into helium at a stable rate. A stars mass determines how long it will stay on the main sequence. The amount of gas and dust available when a star forms determines the mass of the star. More massive stars have large cores and burn through fuel quickly (blue stars). They have shorter lives than smaller stars. Low mass stars have small cores (red stars) and are cooler than high mass stars, so they burn through fuel more slowly. Low mass stars can be on the main sequence for more than 100 billion years.

A star’s core eventually begins to run out of hydrogen A star’s core eventually begins to run out of hydrogen. When this happens, gravity causes the star to collapse inward, heating up the core. The heat gets so intense that hydrogen outside of the core begins to fuse. This causes the outer regions of the star to expand. As the outer regions of the star get farther from the core they cool, and thus appear red.

A star’s core eventually begins to run out of hydrogen A star’s core eventually begins to run out of hydrogen. When this happens, gravity causes the star to collapse inward, heating up the core. The heat gets so intense that hydrogen outside of the core begins to fuse. This causes the outer regions of the star to expand because now the force of energy radiating outward is greater than the inward force of gravity. As the outer regions of the star get farther from the core they cool, and thus appear red.

The star’s hydrogen fuel source is depleted and so the star begins to fuse helium into heavier elements like carbon and oxygen. During helium fusion the star stabilizes and its outer layers shrink and heat up. When no other elements remain to be fused in the star’s core, the energy coming from the star’s interior decreases. The star collapses because the force of gravity is greater than the outward force of energy from the core. As the star collapses it blows off much of its mass leaving only the hot core behind. The gas and dust is illuminated by the remaining core and will last a few thousand years.

The hot dense core that remains after a star has used up all of the hydrogen and helium in its core, collapsed, and blown off much of its mass. These are about the same size as the earth but it incredibly dense. They do NOT undergo fusion, but glow faintly from leftover thermal energy.

When a white dwarf becomes too cool to glow in visible light it is called THIS. None of these have formed yet because it takes about 20 billion years for a white dwarf to cool down and the universe is only about 14 billion years old.

Occur only in the life cycle of a high mass star Occur only in the life cycle of a high mass star. The star’s hydrogen fuel source is depleted and so the star begins to fuse helium into heavier elements like carbon and oxygen. When no other elements remain to be fused in the star’s core, the energy coming from the star’s interior decreases. The star rapidly collapses because the force of gravity is greater than the outward force of energy from the core. The collapse is so dramatic that is causes a violent explosion. There is so much energy that heavier elements like carbon and oxygen are ejected in to space.

The core of a high mass star remains after a supernova spews material in to space. This is the dense remnants of a high-mass star that has exploded in a supernova. Electrons and protons are crushed together by the star’s enormous gravity to form neutrons. These are immensely dense! In fact, a spoonful of matter from a neutron star would weight about a billion tons on Earth. They are only about 25 km across, the size of a large city.

Very massive stars can have incredibly dramatic ends Very massive stars can have incredibly dramatic ends. If a star’s core after a supernova explosion is very massive, its gravitational pull is very strong. Gravity causes the core to collapse beyond the neutron star stage and the collapse of the core continues. The pull of gravity increases and the speed required to escape the star’s core reaches the speed of light. Beyond this point, nothing can escape. This is an object whose surface gravity is so great that even electromagnetic waves traveling at the speed of light cannot escape it. This region of space contains so much matter that it collapses to an infinitely dense point.